The present invention relates generally to multipath radar interferometers, and more particularly, to a method and apparatus for detecting and eliminating signal angle-of-arrival (AOA) errors caused by multipath. More specifically, the present invention substantially improves AOA estimation accuracy and reliability when using super resolution algorithms.
Ship borne search and track radars typically are directed to an emitter by azimuth and elevation coordinates generated from passive measurements in an electronic support (ES) subsystem. The ES system is said to cue the radar. Such cueing avoids time consuming radar target searches over a large volume of space, and hence minimizes the radar""s time-to-range. The ES angle outputs may also be used to drive an emitter tracker.
To be effective in cueing, the ES system must direct the radar to numerous closely spaced targets. This requires azimuth and elevation angle measurement precision comparable to that of the radar. The preferred approach for obtaining this precision as depicted in FIG. 1, is an array of antennas 103 utilized as an interferometer. If the spacing between the outermost interferometer antennas is tens of signal wavelengths, the array can measure signal angle-of-arrival (AOA) to a fraction of a degree. The interferometer can do this over a wide field of view, and over a wide range of frequencies. For both cueing and tracking, the interferometer AOA outputs must be available at an output rate of several Hertz over a period of many seconds.
The interferometer measures the phase difference between pairs of antennas in the array. The signal angle-of-arrival resolution is related to this phase measurement by an accuracy proportional to the spacing d1 101 and d2 102 between the antenna pairs. Hence the requirement for spacings many wavelengths long. But when the antenna pair are more than half a wavelength apart, the phase measured is ambiguous modulo 360xc2x0. A special algorithm, typically tailored to the specific antenna spacings used in the array, must resolve these ambiguities before emitter AOA is found. Conventional ambiguity resolution algorithms (ARA) assume only one signal is present.
Unfortunately, many times in shipboard use this single-signal requirement is not met because of multipath. The presence of two or more simultaneous signals, such as strong specular or diffuse multipath, can cause spurious outputs. Diffuse multipath may produce large phase measurement noise, larger than the ambiguity resolution algorithm was designed to handle robustly. This noise can be so large it causes incorrect ambiguity resolution or xe2x80x9cgross errorsxe2x80x9d. When gross errors occur the resulting spatial errors can be on the order of tens of degrees.
Diffuse multipath affects mainly arrays oriented in azimuth, or with axis 104 parallel to the water""s surface. Specular multipath does not affect a strictly azimuth array. But specular multipath has a profound impact on an interferometer used for elevation angle measurements, i.e. with axis 104 mounted normal to the reflecting surface. And, if the antenna platform is not stabilized, ship pitch and roll assures azimuth and elevation arrays are both affected by specular and diffuse multipath.
Specular multipath induces unacceptable phase errors in an elevation interferometer array by creating a second signal interfering with the direct path one. At low elevation angles these simultaneous interfering signals can be of nearly equal strength Antenna element beam shaping cannot mitigate this phenomenon sufficiently at low elevation angles. Since the ARA and ultimate interferometer AOA generating algorithm are based on measuring phase created by a single plane wave, specular multipath renders them useless for generating elevation measurements to cue the radar or provide angle estimates to a passive emitter tracker.
A straight forward multipath mitigation approach is phase measurement data editing. Specular multipath interference creates a standing wave detected at the array by the varying signal amplitude induced across the different antennas. So one method of data editing is to reject the phase measurements when the amplitude variation exceeds a predetermined threshold. Another method involves histogramming to determine outliers. But such data editing is not viable when cueing a radar or establishing emitter tracks. So many measurements may be rejected that large gaps in elevation angle output, averaging many seconds in length, can result. This is disastrous for cueing and tracking support. Reliable angle estimates at regular, predetermined intervals are required in these applications.
Therefore, the conventional interferometer algorithm must be augmented, especially for elevation processing, in a manner that recovers the direct path phase from multipath corrupted measurements. Currently this augmentation is typically done using a super resolution approach that separates the true signal from it""s specular reflection. In particular, the MUiltiple SIgnal Classification (MUSIC) method, described by Schmidt in xe2x80x9cMultiple Emitter Location and Signal Parameter Estimation,xe2x80x9d Proc. RADC Spectrum Estimation Workshop, October 1979, has been extensively studied for this application. When used for multipath processing the original MUSIC subspace approach had a drawback: the interfering signals must be uncorrelated. This is not the case for multipath. Multipath is the original signal simply shifted in phase and amplitude. But work-a-rounds have been introduced involving decorrelation by spatial averaging, and these work-a-rounds are referred to generically as modified MUSIC.
The spatial averaging is accomplished by implementing interferometer arrays with clusters of elements having certain symmetries. Roy, Paulraj and Kailaith detail the use of such spatial averaging in U.S. Pat. No. 4,965,732, xe2x80x9cMethods and Arrangements for Signal Reception and Parameter Estimation.xe2x80x9d Their approach does not utilize MUSIC, but a super resolution algorithm based on the Estimation of Signal Parameters via Rotational Invariance Techniques (ESPRIT). ESPRIT has also been extensively studied for shipboard interferometer use.
MUSIC and ESPRIT are called subspace techniques because they utilize the fact that all signal vectors are orthogonal to all vectors in the measurement noise subspace. Dividing by the inner product of the candidate signal vector and noise space vectors, these algorithms generate peaks at the true signals. The peak would be at divide-by-zeros if there were no noise. But the presence of measurement error limits their magnitude. The magnitude of the peak is thus a function of the signal-to-noise ratio (SNR). When applying super resolution methods to the interferometer array processing application, where the array antennas are spaced several wavelengths apart, many peaks are generated besides the direct path signal and the multipath signal. Some peaks are caused solely by noise; some peaks appear at the interferometer""s gross error points, or ambiguous AOAs; and some peaks, at higher frequencies, are caused by aliasing. Thus to work robustly, the super resolution approach, whether modified MUSIC, the implementation of ESPRIT disclosed by Roy, or some other subspace technique, must consistently determine which of the many possible outputs is the direct path. The success the method has doing this depends strongly on the direct and reflected signal SNR. When the two signals have comparable SNR, performance can degenerate dramatically for all current subspace approaches.
In the ship board radar cueing and tracking application, comparable SNRs occur often when the emitter is below 10xc2x0 elevation. The reflected signal strength can also be particularly high when the emitter""s signal is horizontally polarized. To assess the performance of subspace algorithms in the critical low elevation region, Litton Advanced Systems (LAS) conducted a field test with the array shown in FIG. 1 during the Fall of 1999. This array has a folded symmetry about horizontal line 100 to allow the use of a particular modified MUSIC spatial averaging method. Use of ESPRIT would have required a different configuration : an array with doublet pairs in subgroups having translational symmetry. But the resulting multipath performance in the low elevation region would have shown essentially the same degradation with any subspace approach using spatial decorrelation.
FIGS. 2a and 2b show the geometry of the test, the break down in conventional single signal ARA, and the result of super resolution processing. The test aircraft 200 in FIG. 2a flew various altitudes 205, e.g.1500, 750 and 200 feet, above the Chesapeake Bay, heading toward a vertical array 201, which was mounted on the water""s edge. The elevation of the aircraft 200 when this particular test started was about 3xc2x0 and, from navigation truth data was determined have the values shown in graph 210. The AOA output 206 from the single-signal interferometer algorithm 202, which attempted to estimate elevation 210 from the phase measurement data, had numerous specular 208 and some diffuse multipath induced errors. Some of the specular errors are indicated by data 207 in the AOA output 206. After processing with the modified MUSIC algorithm 203 in FIG. 2b, the errors 207 (FIG. 2a) were reduced in the elevation angle output 204 (FIG. 2b). But many errors 209 still remained: far too many to reliably cue a radar or passively establish the emitter track. In particular, the gross errors generated by conventional interferometer ambiguity resolution were a problem, and the argumentation of the conventional interferometer ambiguity algorithm by modified MUSIC failed to eliminate a substantial number of these errors. Therefore the ES system would not be able to perform its function in this low angle region using existing super resolution methods.
An object of the present invention is to provide a method and apparatus to predict the next AOA measurement, and compare this prediction with the actual measurement after multipath processing.
Yet another object is to provide a means and method, in this comparison, to determine which of three causes predominated in creating the difference between predicted and actual measurements. The three possible causes are: system thermal and bias error on the phase measurement; ambiguity resolution error due to multipath effects; and prediction error due to target motion.
Still another object of the invention, if a gross error or multipath signal is detected as the estimated AOA, is to substitute the predicted AOA for the measured AOA.
It is a further object of the gross error and multipath post processor to use information from the conventional ambiguity resolution algorithm for the array to determine if the difference in predicted AOA and measured AOA is accounted for by an array gross error rather than a target maneuver.
These and other objects of the present invention are achieved by a method to detect and eliminate interferometer angle of arrival (AOA) estimate errors due to large intermittent phase measurement errors, such as those created by specular and diffuse simultaneous multipath, including measuring signal angle-of-arrival induced phase between sensor pairs and simultaneously processing this phase to detect multiple correlated signals, separating a direct signal path phase contribution to the measured phase from indirect signal path contributions, resolving phase measurement modulo one cycle ambiguities, computing an AOA measurement to a signal source from the direct path resolved phase, saving the AOA measurement, and the time the phase measurements were made, predicting, when a new set of sensor-pair phases are measured, the current AOA from the previously saved AOA measurements and times, where the predicted AOA is made to correspond to the time of the current phase measurements, comparing the predicted AOA to the measured current AOA and rejecting the measured AOA if the comparison falls outside an acceptance window, and substituting the predicted AOA for the measured AOA if the measured AOA is rejected.
The foregoing and other objects of the present invention detect and eliminate interferometer angle of arrival (AOA) estimate errors due to large intermittent phase measurement errors, such as those created by specular and diffuse simultaneous multipath, including measuring means for measuring signal angle-of-arrival induced phase between sensor pairs and simultaneously processing this phase to detect multiple correlated signals, separating means for separating a direct signal path phase contribution to the measured phase from indirect signal path contributions, resolving means for resolving phase measurement modulo one cycle ambiguities, computing means for computing an AOA measurement to a signal source from the direct path resolved phase, saving means for saving the AOA measurement, and the time the phase measurements were made, predicting means for predicting when a new set of sensor-pair phases are measured, the current AOA from the previously saved AOA measurements and times, where the predicted AOA is made to correspond to the time of the current phase measurements, comparing means for comparing the predicted AOA to the measured current AOA and rejecting the measured AOA if the comparison falls outside an acceptance window, and substituting means for substituting the predicted AOA for the measured AOA if the measured AOA is rejected.
This present invention advantageously eliminates the critical deficiencies of current multipath interferometer processing demonstrated in the field test described with respect to FIGS. 2a and 2b, and in particular the present invention substantially improves AOA estimation accuracy and reliability when utilizing super resolution algorithms. The present invention also overcomes the data-gap drawback of data editing methods, especially for emitters at low elevations. The present invention does this by detecting phase processing errors and substituting correct AOA estimates for the corrupt ones. In the preferred implementation, the detection and substitution time extends only slightly the super resolution or data editing processing time. By thus requiring little additional processing time, the present invention allows the interferometer to output accurate angle estimates at the receiver""s emitter-revisit rate for all emitter-array geometries and signal polarizations. The present invention eliminates both the gross errors caused by abnormally large phase noise variance, typically created by diffuse multipath, and the interfering signal errors induced by specular multipath.